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840

The Spinal Column. The Ribs.

spinal marrow is evident in its being a double arch, or strong irregular ring. The spine increases in size towards the bottom, in just proportion, as it has more weight to bear. The articulating surfaces of the spine are so numerous, and so exactly fitted to each other, and are connected by such a number of ligaments of great strength, that the combination of pieces becomes, in reference to motion, a much stronger column than a single bone of the same size would be. It is also remarked that in accidents affecting the spine, the bones are more readily fractured than displaced.

Considering the great number of parts forming the spine, and their nice mutual adaptation, it might be expected that injuries and diseases of the structure would be very frequent. The reverse, however, under natural circumstances, is true; and while many books have been published on the diseases of almost every other part of the body, few comparatively have appeared on spine affections, and these have been chiefly of recent date. One reason of this is that fashions unfavourable to female health began to prevail about the end of the last century, particularly the practice of compressing the chest and abdomen by what was called tightlacing, and a considerable proportion of the young ladies, grew to womanhood with weakened and crooked spines.

1078. The ribs.-Attached to twelve vertebræ in the middle of the back there are the ribs, or bony stretchers of the cavity of the chest, constituting a structure which solves, in the most perfect manner, the difficult mechanical problem of making a cavity with a solid exterior, which shall yet be capable of dilating and contracting itself. Each pair of corresponding ribs may be considered as constituting a hoop, which hangs obliquely down from the place of attachment behind, and so that when the forepart of all the hoops is lifted by the muscles, the cavity of the chest is enlarged. So great is this obliquity, that a straight line, touching the upper edge of the sixth rib behind, would be on a level with the upper edge of the third rib in front.

We have to remark the double connection of the rib behind, first, to the bodies of two adjoining vertebræ, and then to a process or projection from the lower, thus effecting a very steady joint, and yet leaving the necessary freedom of motion; and we observe the fore part of the rib to be joined to the breast-bone by a flexible and elastic cartilage, which allows the degree of motion required there, without the complexity of a joint, and by its elasticity ad mirably guards against the effects of sudden blows or shocks.

The Bones of the Arms.

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The muscles which have their origin on the ribs and their insertion .nto the bones of the arm, afford us an example worth remembering of action and reaction being equal and contrary. When the ribs are fixed, these muscles move the arm; and when the arm is fixed, as by resting on a chair or other object, they with equal force move the ribs. The latter occurrence is seen in the efforts made to breathe during the fits of asthma.

1079. The shoulder-joint is remarkable for combining a great range and variety of motion with considerable strength. The large round head of the shoulder-bone, in order that it may turn freely in all ways, rests upon a shallow cavity or socket of the shoulderblade; and the danger of dislocation from this shallowness of the socket, is guarded against by two strong bony projections from the shoulder-blade above and behind. In order to increase the range of motion to the greatest possible degree, the bone called the shoulder-blade, which carries the socket of the arm, can itself slide about upon the convex exterior of the chest, having its motion Jimited, however, in certain directions by its connection, through the collar-bone or clavicle, with the sternum or bone of the breast.

The scapula, or blade-bone, just spoken of, is remarkable as an illustration of the mechanical rules for combining lightness with strength. It has the strength of the arch from being a little concave, like the “dished wheel” described in Art. 276, p. 154, and its substance is chiefly collected in its borders and spines, with thin plates between, as the strength of a wheel is collected in its rim, spokes, and nave.

The bones of the arms, considered as levers, have the muscles which move them attached very near to the fulcra, and very obliquely, so that these muscles, from working through a short distance, compared with the displacement of the resistances at the extremities, require to be of great strength. It has been calculated that the muscles of the shoulder-joint, in the exertion of supporting a great weight upon the hand, pull with a force of more than a thousand pounds.

Notwithstanding all the securities to the shoulder-joint now described, in the infinite variety of twists, and falls, and accidents to which men in the busy scenes of life are liable, the joint is frequently dislocated; that is, the rounded head of the humerus or arm-bone slips from its socket, with instant loss of power as a consequence.

1080. The os humeri, or bone of the upper arm, is not perfectly

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The Fore-arm and the Wrist.

cylindrical, but like most of the other bones called cylindrical, it has ridges to give strength, on the principle explained in the remarks "on strength of materials." (Art. 287.) These ridges also answer another important purpose. They serve to give a firmer attachment of the muscles to bone. They are most strongly marked in the bones of carnivorous animals.

The elbow-joint is a correct hinge, and so strongly secured that it is rarely dislocated without fracture.

The fore-arm consists of two bones, with a strong membrane between them, binding them together. Its great breadth from this structure affords abundant space for the origin of the many muscles which go to move the hand and fingers : and the very peculiar mode of connection of the two bones gives to man that most useful faculty of turning the hand into what are called the positions of pronation and supination,-exemplified in the action of twisting, or of turning a key in a lock.

The wrist.-The eight small bones, with their numerous joints, forming the wrist, have a signal effect ir. deadening, in regard to the parts above, the shocks or blows which the hand receives, the force being thus distributed over a much larger surface.

The annular ligament is a strong band surrounding the joint, and keeping all the tendons, which pass from the muscles above to the fingers, close to the joint. It answers the purpose of so many fixed pulleys for directing the tendons; without it they would all, on action, start out like bow-strings, producing deformity and weakness.

1081. The pelvis, or strong irregular ring of bone on the upper part of which the spine rests, and from the sides of which the legs descend, forms the central mass of the skeleton. A breadth of bone was wanted here in order to connect the single column of the spine with the lateral columns of the legs, and a circle was the lightest and strongest. If we attempt still farther to conceive how a circle might be modified so as to fit it-for the spine to rest on, for the heads of the thigh-bones to roll in, for muscles to spring from, both above and below, and for the person to be able to sit upon, we shall find that all such anticipations of what was desirable and necessary in a human being, are realized in the most complete manner.

The hip-joint exhibits the perfection of the ball-and-socket articulation. It allows the leg to turn on its axis and to move the foot round in a circle, as well as to have the great range of backward and forward motion, exhibited in the action of walking. When we

The Bones and Joints of the Legs.

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see the elastic, tough, smooth cartilage which lines the deep socket of this joint, and the similar glistening covering of the ball or head of the thigh-bone, and the lubricating synovia, or joint-oil, poured into the cavity by appropriate secretories, and the strong ligaments giving strength to all around, we feel how far the most perfect of human works falls short of the mechanism exhibited in nature.

The femur, or thigh-bone, is remarkable for its two projections near the top, called trochanters,* to which the chief muscles are attached, and which lengthen considerably the levers by which the muscles act. The shaft of the bone is not straight, but has a considerable forward curvature. It might be supposed that this was a structural defect, the bone being a pillar to support a weight; but the bend gives it in reality the strength of the arch, to bear the action of the four large muscles called vasti, which make up the bulk of the thigh.

The knee is a hinge-joint of complicated structure, claiming the most attentive study of the surgeon. The rubbing parts, or those in contact which receive and convey the weight of the body, are flat and shallow, and therefore the joint has little strength from its form ; but it derives security from the numerous and singularly strong ligaments which surround it. The ligaments on the inside of the knees resemble, in two points, the annular ligaments of joints, viz., in having a constant and great strain to bear, and yet in becoming stronger always as the strain increases.

In the knee there is a singular provision of loose cartilages between the ends of the bones. They have been called frictioncartilages, from a supposed relation in use to friction wheels; but their real effect seems to be, to accommodate, in the different positions of the joint, the surfaces of the rubbing bones to each other.

Under the head of Pneumatics, it has been explained that the bones forming the joints are held everywhere in smooth contact, independently of their ligaments, by a constant soft pressure of the atmosphere, amounting in the knee, for instance, to upwards of sixty pounds. (Art. 424.)

1082. The great muscles on the fore part of the thigh are contracted into a single tendon a little above the knee, over and in front of which that tendon has to pass to reach the top of the leg, to which it is attached. The part of the tendon in front of the joint becomes * From Tpoxáw, I turn.

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The Ankle and the Heel.

solid or bone, and forms the patella or knee-pan, often called the pulley of the knee. This peculiarity enables the muscles to act more advantageously, by increasing the distance of the rope from the centre of motion. The patella is, moreover, a sort of shield or protection to the fore-part of this important joint.

The leg below the knee, like the fore-arm already described, has two bones. These, by their ridges and surfaces, present a large space for the origin of the numerous muscles required for the movements of the feet and toes, and they form a compound pillar of greater strength than the same quantity of bone as one shaft would have had. The individual bones also are angular instead of round, hence deriving greater power to resist blows, and giving a more perfect attachment to the powerful muscles of the leg.

The ankle-joint is a perfect hinge of great strength. There is in front of it an annular ligament, by which the greater part of the tendons passing downwards to the foot and toes are kept in their proper places. One of these tendons passes behind and under the bony projection of the inner ankle, in a smooth appropriate groove, exactly as if a little fixed pulley had been placed there.

The heel, by projecting so far backwards, is a lever for those strong muscles to act by, which form the calf of the leg and terminate in the tendo-Achillis. The muscles, by raising this tendon, lift the body, in the actions of standing on the toes, walking, and dancing.

1083. In a graceful human step, the heel is raised a little before the foot is lifted from the ground, as if the foot were part of a wheel rolling forward; and the weight of the body, thus supported by the muscles of the calf of the leg, as just described, rests for the time on the forepart or ball of the foot and the toes. There is at this time a considerable bending of the foot. But where strong wooden shoes are used, or any shoe with a sole so stiff that it will not yield and allow this bending, the heel is not raised at all until the whole foot rises with it, so that the muscles of the calf are scarcely used, and in consequence they soon dwindle remarkably in size. Many of the English farm-servants wear heavy stiff shoes, and in the London markets they may be seen as the drivers of country waggons, with fine robust body and arms, but with legs which are almost spindles, producing an awkward and unmanly gait. The brothers of these men, otherwise employed, are not so mis-shapen; and even they themselves, when they chance to become soldiers, and are trained in military exercises, lose their peculiarity. An example of an

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